Introduction
This invention relates to traffic control systems, and more particularly, to a rangefinder type non-imaging traffic sensor used in such a system.
In U.S. Pat. No. 5,764,163, there is described a non-imaging traffic sensor (NITS). As described therein, the sensor design relied upon filtering, both optical and electronic, to discriminate against shadows. While effective in operation, a drawback of a system employing the sensor was the requirement of a luminaire such as a mercury vapor light or lamp to illuminate the roadway monitored by the system. The luminaire illuminated the scene both day and night. The system also severely filters the sensor signal, greatly reducing signal strength. This, in turn, leads to bandwidth restrictions and reduces resolution time to a level far below that of which the sensor is inherently capable. Additionally, the system requires a spectral filter which is the most expensive item in the optics portion of the system.
As described in the patent, the system utilizes a dual sensor geometry (see FIG. 5 of the patent) to measure vehicle velocity. Because of this, an angle exists between the lines-of-sight (LOS) of the two sensors. Such an angle suggests that a rangefinder principle might be used to determine the height (above the pavement) of a passing disturbance. This would render all the filtering and the daytime artificial illumination unnecessary, since shadows have zero height. A number of advantages now result including lower system cost, an increase in the signal-to-noise ratio (SNR), and enhanced time resolution.
Other patents dealing with traffic flow also address shadow removal. See, for example, U.S. Pat. No. 5,467,402. However, the teachings of this '402 patent require a complicated process of shadow removal requiring information to be maintained concerning the season of the year, time of day, and weather condition information, in addition to information concerning shadow length, luminance, etc.